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Damming the rivers of the Amazon basinMore than a hundred hydropower dams have already been built in the Amazon basin and numerous proposals for further dam constructions are under consideration. The accumulated negative environmental effects of existing dams and proposed dams, if constructed, will trigger massive hydrophysical and biotic disturbances that will affect the Amazon basin's floodplains, estuary and sediment plume. We introduce a Dam Environmental Vulnerability Index to quantify the current and potential impacts of dams in the basin. The scale of foreseeable environmental degradation indicates the need for collective action among nations and states to avoid cumulative, farreaching impacts. We suggest institutional innovations to assess and avoid the likely impoverishment of Amazon rivers.

The Dangers of Reading GloballyThis article is based on a keynote delivered at the 36th IBBY International Congress in Athens, Greece, on August 31, 2018. IBBY members are committed to the potentials offered by global literature for opening minds to multiple ways of living in the world and creating intercultural understanding. Asking readers to read outside their comfort zones, however, can instead hold danger and perpetuate stereotypes and misunderstandings. This article proposes that we can address these dangers through acting on our social responsibilities as bookmakers, readers, and educators to balance individual voice with group responsibility and to determine if our actions could cause harm to readers' understandings of a culture.

A DARK ENERGY CAMERA SEARCH FOR AN OPTICAL COUNTERPART TO THE FIRST ADVANCED LIGO GRAVITATIONAL WAVE EVENT GW150914We report the results of a deep search for an optical counterpart to the gravitational wave (GW) event GW150914, the first trigger from the Advanced LIGO GW detectors. We used the Dark Energy Camera (DECam) to image a 102 deg(2) area, corresponding to 38% of the initial trigger highprobability sky region and to 11% of the revised highprobability region. We observed in the i and z bands at 45, 7, and 24 days after the trigger. The median 5 sigma pointsource limiting magnitudes of our search images are i = 22.5 and z = 21.8 mag. We processed the images through a differenceimaging pipeline using templates from preexisting Dark Energy Survey data and publicly available DECam data. Due to missing template observations and other losses, our effective search area subtends 40 deg(2), corresponding to a 12% total probability in the initial map and 3% in the final map. In this area, we search for objects that decline significantly between days 45 and day 7, and are undetectable by day 24, finding none to typical magnitude limits of i = 21.5, 21.1, 20.1 for object colors (i  z) = 1, 0,  1, respectively. Our search demonstrates the feasibility of a dedicated search program with DECam and bodes well for future research in this emerging field.

A DARK ENERGY CAMERA SEARCH FOR MISSING SUPERGIANTS IN THE LMC AFTER THE ADVANCED LIGO GRAVITATIONALWAVE EVENT GW150914The collapse of a stellar core is expected to produce gravitational waves (GWs), neutrinos, and in most cases a luminous supernova. Sometimes, however, the optical event could be significantly less luminous than a supernova and a direct collapse to a black hole, where the star just disappears, is possible. The GW event GW150914 was detected by the LIGO Virgo Collaboration via a burst analysis that gave localization contours enclosing the Large Magellanic Cloud (LMC). Shortly thereafter, we used DECam to observe 102 deg(2) of the localization area, including 38 deg(2) on the LMC for a missing supergiant search. We construct a complete catalog of LMC luminous red supergiants, the best candidates to undergo invisible core collapse, and collected catalogs of other candidates: less luminous red supergiants, yellow supergiants, blue supergiants, luminous blue variable stars, and WolfRayet stars. Of the objects in the imaging region, all are recovered in the images. The timescale for stellar disappearance is set by the freefall time, which is a function of the stellar radius. Our observations at 4 and 13 days after the event result in a search sensitive to objects of up to about 200 solar radii. We conclude that it is unlikely that GW150914 was caused by the core collapse of a relatively compact supergiant in the LMC, consistent with the LIGO Collaboration analyses of the gravitational waveform as best interpreted as a high mass binary black hole merger. We discuss how to generalize this search for future very nearby corecollapse candidates.

Dark Energy Survey Year 1 results: Cosmological constraints from cosmic shearWe use 26 x 10(6) galaxies from the Dark Energy Survey (DES) Year 1 shape catalogs over 1321 deg(2) of the sky to produce the most significant measurement of cosmic shear in a galaxy survey to date. We constrain cosmological parameters in both the flat Lambda CDM and the wCDM models, while also varying the neutrino mass density. These results are shown to be robust using two independent shape catalogs, two independent photoz calibration methods, and two independent analysis pipelines in a blind analysis. We find a 3.5% fractional uncertainty on sigma(8) (Omega(m)/0.3)(0.5) = 0.782( 0.027)(+0.027) at 68% C. L., which is a factor of 2.5 improvement over the fractional constraining power of our DES Science Verification results. In wCDM, we find a 4.8% fractional uncertainty on sigma(8) (Omega(m)/0.3)(0.5) = 0.777(0.038)(+0.036) and a dark energy equationofstate w = 0.95(0.39)(+0.33) . We find results that are consistent with previous cosmic shear constraints in sigma(8)Omega(m), and we see no evidence for disagreement of our weak lensing data with data from the cosmic microwave background. Finally, we find no evidence preferring a wCDM model allowing w not equal 1. We expect further significant improvements with subsequent years of DES data, which will more than triple the sky coverage of our shape catalogs and double the effective integrated exposure time per galaxy.

Dark Energy Survey year 1 results: Cosmological constraints from galaxy clustering and weak lensingWe present cosmological results from a combined analysis of galaxy clustering and weak gravitational lensing, using 1321 deg(2) of griz imaging data from the first year of the Dark Energy Survey (DES Y1). We combine three twopoint functions: (i) the cosmic shear correlation function of 26 million source galaxies in four redshift bins, (ii) the galaxy angular autocorrelation function of 650,000 luminous red galaxies in five redshift bins, and (iii) the galaxyshear crosscorrelation of luminous red galaxy positions and source galaxy shears. To demonstrate the robustness of these results, we use independent pairs of galaxy shape, photometricredshift estimation and validation, and likelihood analysis pipelines. To prevent confirmation bias, the bulk of the analysis was carried out while "blind" to the true results; we describe an extensive suite of systematics checks performed and passed during this blinded phase. The data are modeled in flat Lambda CDM and wCDM cosmologies, marginalizing over 20 nuisance parameters, varying 6 (for Lambda CDM) or 7 (for wCDM) cosmological parameters including the neutrino mass density and including the 457 x 457 element analytic covariance matrix. We find consistent cosmological results from these three twopoint functions and from their combination obtain S8 equivalent to sigma(8) (Omega(m)/0.3)(0.5) = 0.773(0.020)(+0.026) and Omega(m) = 0.267(0.017)(+0.030) for Lambda CDM; for wCDM, we find S8 = 0.782(0.024)(+0.036) , Omega(m) = 0.284(0.030)(+0.033), and w = 0.82(0.20)(+0.21) at 68% C.L. The precision of these DES Y1 constraints rivals that from the Planck cosmic microwave background measurements, allowing a comparison of structure in the very early and late Universe on equal terms. Although the DES Y1 bestfit values for S8 and Omega(m) are lower than the central values from Planck for both Lambda CDM and wCDM, the Bayes factor indicates that the DES Y1 and Planck data sets are consistent with each other in the context of Lambda CDM. Combining DES Y1 with Planck, baryonic acoustic oscillation measurements from SDSS, 6dF, and BOSS and type Ia supernovae from the Joint Lightcurve Analysis data set, we derive very tight constraints on cosmological parameters: S8 = 0.802 +/ 0.012 and Omega(m) = 0.298 +/ 0.007 in Lambda CDM and w = 1.00(0.04)(+0.05) in wCDM. Upcoming Dark Energy Survey analyses will provide more stringent tests of the Lambda CDM model and extensions such as a timevarying equation of state of dark energy or modified gravity.

Dark Energy Survey Year 1 Results: Crosscorrelation between Dark Energy Survey Y1 galaxy weak lensing and South Pole Telescope+Planck CMB weak lensingWe crosscorrelate galaxy weak lensing measurements from the Dark Energy Survey (DES) yearone data with a cosmic microwave background (CMB) weak lensing map derived from South Pole Telescope (SPT) and Planck data, with an effective overlapping area of 1289 deg(2). With the combined measurements from four source galaxy redshift bins, we obtain a detection significance of 5.8 sigma. We fit the amplitude of the correlation functions while fixing the cosmological parameters to a fiducial Lambda CDM model, finding A = 0.99 +/ 0.17. We additionally use the correlation function measurements to constrain shear calibration bias, obtaining constraints that are consistent with previous DES analyses. Finally, when performing a cosmological analysis under the Lambda CDM model, we obtain the marginalized constraints of Omega(m) = 0.261(0.051)(+0.070) and S8 = sigma(8)root Omega(m)/0.3 = 0.660(0.100)(+0.085). These measurements are used in a companion work that presents cosmological constraints from the joint analysis of twopoint functions among galaxies, galaxy shears, and CMB lensing using DES, SPT, and Planck data.

Dark Energy Survey Year 1 results: crosscorrelation redshifts – methods and systematics characterizationWe use numerical simulations to characterize the performance of a clusteringbased method to calibrate photometric redshift biases. In particular, we crosscorrelate the weak lensing source galaxies from the Dark Energy Survey Year 1 sample with redMaGiC galaxies (luminous red galaxies with secure photometric redshifts) to estimate the redshift distribution of the former sample. The recovered redshift distributions are used to calibrate the photometric redshift bias of standard photoz methods applied to the same source galaxy sample. We apply the method to two photoz codes run in our simulated data: Bayesian Photometric Redshift and Directional Neighbourhood Fitting. We characterize the systematic uncertainties of our calibration procedure, and find that these systematic uncertainties dominate our error budget. The dominant systematics are due to our assumption of unevolving bias and clustering across each redshift bin, and to differences between the shapes of the redshift distributions derived by clustering versus photozs. The systematic uncertainty in the mean redshift bias of the source galaxy sample is Delta z less than or similar to 0.02, though the precise value depends on the redshift bin under consideration. We discuss possible ways to mitigate the impact of our dominant systematics in future analyses.

Dark Energy Survey Year 1 results: curvedsky weak lensing mass mapWe construct the largest curvedsky galaxy weak lensing mass map to date from the DES firstyear (DES Y1) data. The map, about 10 times larger than the previous work, is constructed over a contiguous approximate to 1500 deg(2), covering a comoving volume of approximate to 10 Gpc(3). The effects of masking, sampling, and noise are tested using simulations. We generate weak lensing maps from two DES Y1 shear catalogues, METACALIBRATION and IM3SHAPE, with sources at redshift 0.2 < z < 1.3, and in each of four bins in this range. In the highest signaltonoise map, the ratio between the mean signal to noise in the Emode map and the Bmode map is similar to 1.5 (similar to 2) when smoothed with a Gaussian filter of sigma(G) = 30 (80) arcmin. The second and third moments of the convergence kappa in the maps are in agreement with simulations. We also find no significant correlation of kappa with maps of potential systematic contaminants. Finally, we demonstrate two applications of the mass maps: (1) crosscorrelation with different foreground tracers of mass and (2) exploration of the largest peaks and voids in the maps.

Dark Energy Survey Year 1 Results: Detection of Intracluster Light at Redshift ∼ 0.25Using data collected by the Dark Energy Survey (DES), we report the detection of intracluster light (ICL) with similar to 300 galaxy clusters in the redshift range of 0.20.3. We design methods to mask detected galaxies and stars in the images and stack the cluster light profiles, while accounting for several systematic effects (sky subtraction, instrumental pointspread function, cluster selection effects, and residual light in the ICL raw detection from background and cluster galaxies). The methods allow us to acquire high signaltonoise measurements of the ICL and central galaxies (CGs), which we separate with radial cuts. The ICL appears as faint and diffuse light extending to at least 1 Mpc from the cluster center, reaching a surface brightness level of 30 mag arcsec(2). The ICL and the cluster CG contribute 44% +/ 17% of the total cluster stellar luminosity within 1 Mpc. The ICL color is overall consistent with that of the cluster red sequence galaxies, but displays the trend of becoming bluer with increasing radius. The ICL demonstrates an interesting selfsimilarity featurefor clusters in different richness ranges, their ICL radial profiles are similar after scaling with cluster R200(m), and the ICL brightness appears to be a good tracer of the cluster radial mass distribution. These analyses are based on the DES redMaPPer cluster sample identified in the first year of observations.

Dark Energy Survey year 1 results: Galaxy clustering for combined probesWe measure the clustering of DES year 1 galaxies that are intended to be combined with weak lensing samples in order to produce precise cosmological constraints from the joint analysis of largescale structure and lensing correlations. Twopoint correlation functions are measured for a sample of 6.6 x 10(5) luminous red galaxies selected using the REDMAGIC algorithm over an area of 1321 square degrees, in the redshift range 0.15 < z < 0.9, split into five tomographic redshift bins. The sample has a mean redshift uncertainty of sigma(z)/(1 + z) = 0.017. We quantify and correct spurious correlations induced by spatially variable survey properties, testing their impact on the clustering measurements and covariance. We demonstrate the sample's robustness by testing for stellar contamination, for potential biases that could arise from the systematic correction, and for the consistency between the twopoint autoand crosscorrelation functions. We show that the corrections we apply have a significant impact on the resultant measurement of cosmological parameters, but that the results are robust against arbitrary choices in the correction method. We find the linear galaxy bias in each redshift bin in a fiducial cosmology to be b(sigma(8)/0.81) vertical bar(z=0.24) =1.40 +/ 0.07, b(sigma(8)/0.81) vertical bar(z=0.38) = 1.60 +/ 0.05, (sigma(8)/0.81) vertical bar(z=0.53) = 1.60 +/ 0.04 for galaxies with luminosities L/L* > 0.5, b(sigma(8)/0.8) vertical bar(z=0.68) = 1.93 +/ 0.04 for L/L* > 1 and b(sigma(8)/0.81) vertical bar(z=0.83) = 1.98 +/ 0.07 for L/L* > 1.5, broadly consistent with expectations for the redshift and luminosity dependence of the bias of red galaxies. We show these measurements to be consistent with the linear bias obtained from tangential shear measurements.

Dark Energy Survey year 1 results: Galaxygalaxy lensingWe present galaxygalaxy lensing measurements from 1321 sq. deg. of the Dark Energy Survey (DES) Year 1 (Y1) data. The lens sample consists of a selection of 660,000 red galaxies with highprecision photometric redshifts, known as redMaGiC, split into five tomographic bins in the redshift range 0.15 < z < 0.9. We use two different source samples, obtained from the METACALIBRATION (26 million galaxies) and IM3SHAPE (18 million galaxies) shear estimation codes, which are split into four photometric redshift bins in the range 0.2 < z < 1.3. We perform extensive testing of potential systematic effects that can bias the galaxygalaxy lensing signal, including those from shear estimation, photometric redshifts, and observational properties. Covariances are obtained from jackknife subsamples of the data and validated with a suite of lognormal simulations. We use the shearratio geometric test to obtain independent constraints on the mean of the source redshift distributions, providing validation of those obtained from other photoz studies with the same data. We find consistency between the galaxy bias estimates obtained from our galaxygalaxy lensing measurements and from galaxy clustering, therefore showing the galaxymatter crosscorrelation coefficient r to be consistent with one, measured over the scales used for the cosmological analysis. The results in this work present one of the three twopoint correlation functions, along with galaxy clustering and cosmic shear, used in the DES cosmological analysis of Y1 data, and hence the methodology and the systematics tests presented here provide a critical input for that study as well as for future cosmological analyses in DES and other photometric galaxy surveys.

Dark Energy Survey year 1 results: Joint analysis of galaxy clustering, galaxy lensing, and CMB lensing twopoint functionsWe perform a joint analysis of the auto and crosscorrelations between three cosmic fields: the galaxy density field, the galaxy weak lensing shear field, and the cosmic microwave background (CMB) weak lensing convergence field. These three fields are measured using roughly 1300 sq. deg. of overlapping optical imaging data from first year observations of the Dark Energy Survey (DES) and millimeterwave observations of the CMB from both the South Pole Telescope SunyaevZel'dovich survey and Planck. We present cosmological constraints from the joint analysis of the twopoint correlation functions between galaxy density and galaxy shear with CMB lensing. We test for consistency between these measurements and the DESonly twopoint function measurements, finding no evidence for inconsistency in the context of flat Lambda CDM cosmological models. Performing a joint analysis of five of the possible correlation functions between these fields (excluding only the CMB lensing autospectrum) yields S8 sigma(8) root Omega(m)/0.3 = 0.782(0.025)(+0.019) and Omega(m) = 0.260(0.019)(+0.029). We test for consistency between these five correlation function measurements and the Planckonly measurement of the CMB lensing autospectrum, again finding no evidence for inconsistency in the context of flat Lambda CDM models. Combining constraints from all six twopoint functions yields S8 = 0.776(0.021)(+0.014) and Omega(m) = 0.271(0.016)(+0.022). These results provide a powerful test and confirmation of the results from the first year DES jointprobes analysis.

Dark Energy Survey Year 1 results: measurement of the baryon acoustic oscillation scale in the distribution of galaxies to redshift 1We present angular diameter distance measurements obtained by locating the baryon acoustic oscillations (BAO) scale in the distribution of galaxies selected from the first year of Dark Energy Survey data. We consider a sample of over 1.3 million galaxies distributed over a footprint of 1336 deg(2) with 0.6 < z(photo) < 1 and a typical redshift uncertainty of 0.03(1 + z). This sample was selected, as fully described in a companion paper, using a colour/magnitude selection that optimizes tradeoffs between number density and redshift uncertainty. We investigate the BAO signal in the projected clustering using three conventions, the angular separation, the comoving transverse separation, and spherical harmonics. Further, we compare results obtained from templatebased and machinelearning photometric redshift determinations. We use 1800 simulations that approximate our sample in order to produce covariance matrices and allow us to validate our distance scale measurement methodology. We measure the angular diameter distance, DA, at the effective redshift of our sample divided by the true physical scale of the BAO feature, r(d). We obtain close to a 4 per cent distance measurement of DA (z(eff )= 0.81)/r(d) = 10.75 +/ 0.43. These results are consistent with the flat A cold dark matter concordance cosmological model supported by numerous other recent experimental results.

Dark Energy Survey Year 1 Results: redshift distributions of the weaklensing source galaxiesWe describe the derivation and validation of redshift distribution estimates and their uncertainties for the populations of galaxies used as weaklensing sources in the Dark Energy Survey (DES) Year 1 cosmological analyses. The Bayesian Photometric Redshift (BPZ) code is used to assign galaxies to four redshift bins between z approximate to 0.2 and approximate to 1.3, and to produce initial estimates of the lensingweighted redshift distributions n(PZ)(i)(z) proportional to d(n)(i)/dz for members of bin i. Accurate determination of cosmological parameters depends critically on knowledge of n(i), but is insensitive to bin assignments or redshift errors for individual galaxies. The cosmological analyses allow for shifts n(i)(z) = n(PZ)(i)(z  Delta z(i)) to correct the mean redshift of n(i)(z) for biases in n(PZ)(i). The Delta z(i) are constrained by comparison of independently estimated 30band photometric redshifts of galaxies in the Cosmic Evolution Survey (COSMOS) field to BPZ estimates made from the DES griz fluxes, for a sample matched in fluxes, preseeing size, and lensing weight to the DES weaklensing sources. In companion papers, the Delta z(i) of the three lowest redshift bins are further constrained by the angular clustering of the source galaxies around red galaxies with secure photometric redshifts at 0.15 < z < 0.9. This paper details the BPZ and COSMOS procedures, and demonstrates that the cosmological inference is insensitive to details of the n(i)(z) beyond the choice of Delta z(i). The clustering and COSMOS validation methods produce consistent estimates of Delta z(i) in the bins where both can be applied, with combined uncertainties of sigma(i)(Delta z) = 0.015, 0.013, 0.011, and 0.022 in the four bins. Repeating the photoz procedure instead using the Directional Neighbourhood Fitting algorithm, or using the n(i)(z) estimated from the matched sample in COSMOS, yields no discernible difference in cosmological inferences.

Dark Energy Survey Year 1 results: the effect of intracluster light on photometric redshifts for weak gravitational lensingWe study the effect of diffuse intracluster light on the critical surface mass density estimated from photometric redshifts of lensing source galaxies, and the resulting bias in a weak lensing measurement of galaxy cluster mass. Under conservative assumptions, we find the bias to he negligible for imaging surveys like the Dark Energy Survey with a recommended scale cut of >= 200 kpc distance from cluster centres. For significantly deeper lensing source galaxy catalogues from present and future surveys like the Large Synoptic Survey Telescope program, more conservative scale and source magnitude cuts or a correction of the effect may be necessary to achieve percent level lensing measurement accuracy, especially at the massive end of the cluster population.

Dark Energy Survey Year 1 Results: Tomographic crosscorrelations between Dark Energy Survey galaxies and CMB lensing from South Pole Telescope+PlanckWe measure the crosscorrelation between REDMAGIC galaxies selected from the Dark Energy Survey (DES) year 1 data and gravitational lensing of the cosmic microwave background (CMB) reconstructed from South Pole Telescope (SPT) and Planck data over 1289 deg(2). When combining measurements across multiple galaxy redshift bins spanning the redshift range of 0.15 < z < 0.90, we reject the hypothesis of no correlation at 19.9 sigma significance. When removing smallscale data points where thermal SunyaevZel'dovich signal and nonlinear galaxy bias could potentially bias our results, the detection significance is reduced to 9.9 sigma. We perform a joint analysis of galaxyCMB lensing crosscorrelations and galaxy clustering to constrain cosmology, finding Omega m = 0.276(0.030)(+0.029) and S8 = sigma(8) root Omega(m)/0.3 = 0.800(0.094)(+0.090). we also perform two alternate analyses aimed at constraining only the growth rate of cosmic structure as a function of redshift, finding consistency with predictions from the concordance Lambda CDM model. The measurements presented here are part of a joint cosmological analysis that combines galaxy clustering, galaxy lensing and CMB lensing using data from DES, SPT and Planck.

Dark Energy Survey Year 1 results: validation of weak lensing cluster member contamination estimates from P(z) decompositionWeak lensing source galaxy catalogues used in estimating the masses of galaxy clusters can be heavily contaminated by cluster members, prohibiting accurate mass calibration. In this study, we test the performance of an estimator for the extent of cluster member contamination based on decomposing the photometric redshift P(z) of source galaxies into contaminating and background components. We perform a full scale mock analysis on a simulated sky survey approximately mirroring the observational properties of the Dark Energy Survey Year One observations (DES Y1), and find excellent agreement between the true number profile of contaminating cluster member galaxies in the simulation and the estimated one. We further apply the method to estimate the cluster member contamination for the DES Y1 redMaPPer cluster mass calibration analysis, and compare the results to an alternative approach based on the angular correlation of weak lensing source galaxies. We find indications that the correlation based estimates are biased by the selection of the weak lensing sources in the cluster vicinity, which does not strongly impact the P(z) decomposition method. Collectively, these benchmarks demonstrate the strength of the P(z) decomposition method in alleviating membership contamination and enabling highly accurate cluster weak lensing studies without broad exclusion of source galaxies, thereby improving the total constraining power of cluster mass calibration via weak lensing.

The Dark Energy Survey: more than dark energy – an overviewThis overview paper describes the legacy prospect and discovery potential of the Dark Energy Survey (DES) beyond cosmological studies, illustrating it with examples from the DES early data. DES is using a widefield camera (DECam) on the 4 m Blanco Telescope in Chile to image 5000 sq deg of the sky in five filters (grizY). By its completion, the survey is expected to have generated a catalogue of 300 million galaxies with photometric redshifts and 100 million stars. In addition, a timedomain survey search over 27 sq deg is expected to yield a sample of thousands of Type Ia supernovae and other transients. The main goals of DES are to characterize dark energy and dark matter, and to test alternative models of gravity; these goals will be pursued by studying largescale structure, cluster counts, weak gravitational lensing and Type Ia supernovae. However, DES also provides a rich data set which allows us to study many other aspects of astrophysics. In this paper, we focus on additional science with DES, emphasizing areas where the survey makes a difference with respect to other current surveys. The paper illustrates, using early data (from 'Science Verification', and from the first, second and third seasons of observations), what DES can tell us about the Solar system, the Milky Way, galaxy evolution, quasars and other topics. In addition, we show that if the cosmological model is assumed to be I >+cold dark matter, then important astrophysics can be deduced from the primary DES probes. Highlights from DES early data include the discovery of 34 transNeptunian objects, 17 dwarf satellites of the Milky Way, one published z > 6 quasar (and more confirmed) and two published superluminous supernovae (and more confirmed).

Dark Energy Surveyed Year 1 results: calibration of cluster miscentring in the redMaPPer cataloguesThe centre determination of a galaxy cluster from an optical cluster finding algorithm can be offset from theoretical prescriptions or Nbody definitions of its host halo centre. These offsets impact the recovered cluster statistics, affecting both richness measurements and the weak lensing shear profile around the clusters. This paper models the centring performance of the redMaPPer cluster finding algorithm using archival Xray observations of redMaPPerselected clusters. Assuming the Xray emission peaks as the fiducial halo centres, and through analysing their offsets to the redMaPPer centres, we find that similar to 75 +/ 8 per cent of the redMaPPer clusters are well centred and the miscentred offset follows a Gamma distribution in normalized, projected distance. These miscentring offsets cause a systematic underestimation of cluster richness relative to the wellcentred clusters, for which we propose a descriptive model. Our results enable the DES Y1 cluster cosmology analysis by characterizing the necessary corrections to both the weak lensing and richness abundance functions of the DES Y1 redMaPPer cluster catalogue.